Methods and apparatus of this kind are used for pretreatment of organic substrates, which after having passed a hydrolizer (a device for thermal pressure hydrolysis) are fed into a fermenter, for instance a biogas or biofuel plant.
Thermal pressure hydrolysis uses a technology called “steam explosion,” which is known from biogas and biofuel plants. “Steam explosion” is a technical process in which the input material is heated up to 300° C., preferably 150° C. to 200° C., and exposed to a pressure of 3 bar up to 20 bar. This pressure-temperature state is upheld for a certain period of time, after which the substrate is suddenly depressurized to atmospheric pressure. Due to this depressurization shock the cell substance is completely broken down. All of the organic substance is then present in liquified form for further processing.
The initially inhomogeneous substrate mixture (for instance, energy crops, harvesting waste etc.) is transformed into a homogeneous pulp having the following properties: cellulose is set free; crusts of hemicellulose-lignin complexes are broken down; hemicellulose is cooked; yeast, mildew and other undesirable microorganisms are destroyed; the substrate is sterilized; and fibrous matter is destabilized.
Prior to further substrate processing, for instance in a bio-gas plant, “steam explosion” thus takes care of the process steps of hydrolysis and homogenization. Fermentation conditions may thus be specifically optimized for processes of acido/acetogenesis and methanogenesis.
The result of such pretreatment is an increased substrate yield and improved product quality, in the case of a biogas plant a higher substrate decomposition rate with increased gas production and improved gas quality. Typically, specific methane content (CH4) is increased while noxious hydrogen sulfide content (H2S) is reduced.
U.S. Patent No. 2003/0121851 A1 describes a method and apparatus for treating biologically degradable organic waste. Before the organic waste is submitted to thermal pressure hydrolysis an alkaline solution (KOH) is added to the substrate and the substrate is subjected to temperatures of 170° C. to 225° C. and correlated vapour pressure in the hydrolizer. Solid/liquid separation is then carried out. Prior to treatment the substrate may be preheated in a tank by recycled steam from the hydrolizer.
From WO 2008/011839 A2 there has for instance become known a plant for continuous and discontinuous hydrolysis of organic substrates. The plant essentially comprises a shredder for the inhomogeneous organic substrate, from which the substrate is fed to a metering charger for the hydrolizer. After treatment of the substrate in the hydrolizer it is conveyed via an “overshooting pipe” into a flash tank, from which an exhaust gas line leads to a condenser and a substrate line leads to a fermenter. The exhaust gases are fed into a steam condenser, which is water-cooled, and the condensate obtained by this step is recycled to the flash tank. The substrate line to the fermenter contains a heat exchanger whose waste heat is supplied via an external heat exchanger circuit to a heat exchanger used as preheating device, which will heat the input substrate coming from the shredder.
From SU 1620487 A1 there is known a hydrolizer having two concentric screw conveyors in a cylindrical housing, between which a drum screen is disposed. The organic material enters an outer cylindrical annular chamber via a feeder pipe and is compressed by the first screw conveyor, with superheated steam being fed into the outer annular chamber via a steam line. Then the material arrives in the inner hollow space where it is transported in reverse direction to an exit opening by the second screw conveyor.
The known methods and apparatus suffer from the disadvantage of not being energetically optimized and having a relatively complex structure.
In this context there has become known from EP 2 177 280 an apparatus for discontinuous hydrolysis of organic substances, which comprises the following components: a liquid-filled preconditioning tank for receiving solid floatable organic substrates, with an agitator and a steam distributor unit, configured as a special jet stock for creating a flotation effect; a screw conveyor for taking organic substrate from a floating mat building up on the surface, with an integrated sieve unit and a recirculation line for recirculating the filtrate; a charger unit with a pressure vessel (blow gun) and a charger gate and an additional valve-controlled connecting line to the hydrolizer; a transfer pump for taking liquid from the preconditioning tank and feeding it to the charger unit; a hydrolizer with agitator for carrying out thermal pressure hydrolysis; a valve-controlled depressurizing unit with a pressure baffle, a cyclone; and a flash tank with integrated heat exchanger.
The apparatus known from EP 2 177 280 is suitable in particular for the processing of substrates and substrate mixtures with a certain liquid content or admixture of liquid, where the floatable solid components are separated by rinsing or flotation prior to charging the hydrolizer. It is a disadvantage that reliable balancing of substrate intake is not possible due to the uncontrolled intake of liquid of the solid component during the pulping process.